Prolactinomas are the most common pituitary adenomas, occurring primarily in women of reproductive age. Manifestations of hyperprolactinemia include infertility in women and impotence in men. Estrogens stimulate prolactin gene expression and release, and promote lactotroph proliferation, but their role in the development of prolactinomas remain unclear. The recent discovery of estrogen receptor variants and a novel estrogen receptor raises questions regarding their presence and function in human prolactinomas. We recently identified two distinct cell populations in human prolactinoma cell cultures. The majority of cells are lactotrophs, as evident by their expression and release of prolactin. These cells also express estrogen receptors. A morphologically distinct subpopulation of cells produces prolactin regulating factor (PRF), as judged by increased prolactin gene expression in cocultured rat lactotrophs. It is our hypothesis that different forms of the estrogen receptor and/or a subpopulation of PRF-producing cells support the development of human prolactinomas. Our objectives are to: a) characterize estrogen receptors and estrogenic actions in cultured human prolactinoma cells, b) investigate PRF-producing cells in these cultures, c) establish a human prolactinoma cell line. This research will help to understand the etiology of human prolactinomas.